Some hybrid vehicles use a primary power source for the majority of the vehicle's propulsion needs and supplement that primary power source with an auxiliary power source, as needed. An auxiliary power source can benefit a hybrid vehicle in certain driving conditions, such as scaling large grades, i.e., steep inclines and declines.
In order to utilize the auxiliary power source, the auxiliary power source requires electric power from a power storage device. When climbing a significant grade, the auxiliary power source may diminish the stored charge of the power storage device more quickly, while operating in an electric-only mode or a hybrid mode.